The present invention relates to equipment for reducing emissions during cleaning of air atomization spray guns for paint. More particularly, the invention concerns a device for capturing and condensing aspirated cleaning solvent that is sprayed from the spray gun during cleaning.
One common method of applying paint is by pneumatic spray equipment, such as a hand-held spray gun or pressure pot. Air atomization spray equipment produced a pattern of fine, uniform droplets. The paint is drawn up into the spray gun and forced out of an orifice in the spray gun nozzle. The nozzle comprises a fluid tip, having an orifice to deliver the paint, and an air cap that surrounds the fluid tip. The air cap delivers a pattern of atomization air around the discharged stream of paint. The paint is atomized by the pressurized streams of air impinging the paint from different points around the air cap, thereby breaking the paint stream into discrete paint droplets. The air cap also delivers jets of pattern shaping air against diametrically opposite sides of the atomized paint spray. Breaking the paint into spray droplets allows application of a smooth and even coating of paint onto the surface being coated.
Usually, the same equipment is used to apply any number of different paint formulations and colors consecutively onto different articles needing to be painted. The spray equipment must be cleaned before it is used for a different paint in order to prevent contamination of the new paint with the residue of the previous paint that is left in and on the spraygun. Such contamination could result in, for example, cratering of the coating if the two formulations are incompatible, or a color shift if the two formulations are different in color. Organic solvents are commonly used to clean out the spray gun and other spray equipment. The cleaning operation can be simply spraying solvent through the spray gun. Low molecular weight solvents such as acetone, methyl ethyl ketone, isopropyl alcohol, toluene, xylene, and butyl acetate, used alone or in combination, readily wash out the paint and also evaporate fast so that the equipment is clean and dry for application of the next paint.
A significant drawback to this simple procedure of spraying solvent through to clean out the spray gun is venting of the cleaning solvent to the atmosphere. Emissions of volatile organic compounds are rigorously controlled by both state and federal governments. Facility permits limit how much solvent can be emitted and require record keeping of all emissions of volatile organic compounds, whether as a specific permit condition or as a general need to show that the limits have been respected. In addition, some of the best cleaning solvents (methanol, methyl ethyl ketone, toluene, xylene, and ethylene glycol ethers) are further regulated as so-called hazardous air pollutants (HAP) under the Federal Clean Air Act. It would thus be advantageous to capture the sprayed cleaning solvent so as to prevent its release to the environment. Finally, if the cleaning solvent is captured and recondensed, it can be recycled, for instance by distilling it from the paint sludge and reusing it as cleaning solvent. Recycling the solvent instead of emitting it to the atmosphere thus has both ecological and economic benefits.
U.S. Pat. No. 4,934,393 to Lighthall et al. discloses an apparatus for condensing and collecting used cleaning solvent from cleaning a spray gun. The Lighthall apparatus requires a negative pressure generator connected to a source of compressed air. The negative pressure pulls the air through the filter condenser of the apparatus. Lighthall describes condensers as preferably containing fine mesh brass wool. In the preferred embodiment, the vapors are also condensed in concentric cylindrical passageways.
An alternative, passive spray gun cleaning apparatus, which operates by a simplified mechanism is desirable. It is this objective to which the present invention is directed.